1. Field of the Invention
The present invention generally relates to a conversion control circuit for an image display apparatus, and more particularly to a conversion control circuit for controlling a ratio of brightness of an ultrasonic image of a human body to that of a color image such as blood-flow patterns. These colored patterns are superimposed with the ultrasonic image (black-and-white image) on the same monitor screen.
2. Description of Related Art
An ultrasonic blood-flow imaging apparatus is available as an apparatus for visualizing blood-flow information of a body under examination for diagnosis based on reflected echoes of ultrasonic waves transmitted toward and reflected from the interior of a body under examination.
In this imaging apparatus, the blood-flow information of the body is obtained by utilizing the frequency shift, i.e., Doppler shift, of the ultrasonic waves reflected from the blood flow. The blood-flow information is converted into predetermined colors corresponding to changes in direction and the average velocity of the blood flow and is superimposed on an ultrasonic B or M mode image (black-and-white image) of the body. For example, a blood flow flowing toward an ultrasonic probe is displayed in red, and blood flowing in a direction away from the ultrasonic probe is displayed in blue.
Such an imaging apparatus uses a color display cathode ray tube (CRT) as a monitor and employs the RGB drive method. According to the RGB drive method, in a dynamic range allowing a good white balance of the monitor, brightness Y is represented as: EQU Y.apprxeq.0.3 VR+0.6 VG+0.1 VB
where VR, VG, and VB are input voltages to the RGB color monitor. A white color is obtained when VR=VG=VB.
In the conventional blood-flow information display apparatus, blood-flow information (color image) superimposed on an ultrasonic image (black-and-white image) of a body under examination cannot be visually distinguished from the black-and-white image. This is probably caused by the following reasons.
FIG. 1 is an illustration of a VR-VG-VB orthogonal coordinate system, and FIG. 2 is an illustration of a displayed color.
Referring to FIG. 1, assuming that the respective maximum input voltages of an RGB color monitor are VRmax, VGmax, and VBmax, points having chromaticity brightnesses displayable on the RGB color monitor exist on and inside a cube having sides Vmax. Planes 1, 2, 3, and 4 are equi-bright planes of every 0.2 steps when the white brightness of a point (VRmax, VGmax, VBmax) is 1, and origin 0 (0, 0, 0) is 0. The gradation of a black-and-white image (ultrasonic B or M mode image) moves on diagonal line 5 connecting origin 0 and a maximum brightness point.
Referring to FIG. 2, the maximum brightness of a point in a region (blue or red region) indicated by hatched lines 10 and having a high saturation is lower than that of white color. Therefore, when a color belonging to this region is used for a gradation of a color image (blood-flow image), the color image cannot be clearly discriminated from the black-and-white image, because of a brightness imbalance between the black-and-white image and the color image.
A problem in blood-flow image display by the conventional ultrasonic blood-flow imaging apparatus has been described. Such a problem appears in other image display systems of various types for superimposing a color image on a black-and-white image.
As described above, in the conventional display system, when a color image is superimposed on a black-and-white image, the color image cannot be seen as clearly as can black-and-white image. An example of the conventional ultrasonic imaging apparatus of this type is disclosed in, e.g., European patent application No. 0100094.
It is an object of the present invention to provide a conversion control circuit for an image display apparatus which can distinctly display a color image distinguishably with respect to a black-and-white image when the color and black-and-white images are simultaneously displayed on the same monitor screen.